1. Field of the Invention
The present invention relates to a sewing machine, and more particularly, it relates to a sewing machine having a balance.
2. Description of the Prior Art
A sewing machine having a balance is known in general. This balance has a thread engaging part. The balance is so structured that the thread engaging part captures and pulls a needle thread when forwardly moving from left to right. In this case, the thread engaging part of the balance engages with the needle thread passing through thread receiving openings of a pair of thread guide plates for pulling the same.
When a sewing machine operating switch of the conventional sewing machine having the aforementioned structure is moved to OFF for stopping the sewing machine, however, a motor is generally stopped when a needle is located on a top dead center. When the needle is located on the top dead center, the thread engaging part of the balance is generally located on an intermediate position of the passage for the forward movement beyond the thread guide plates. In other words, the motor is generally stopped regardless of the position of the thread engaging part of the balance.
Therefore, when the sewing machine operating switch is moved to ON after the needle thread is exchanged, for example, the sewing machine starts the first operating cycle without capturing and pulling the needle thread by the thread engaging part of the balance. Thus, the needle thread has no prescribed slack in the first operating cycle of the sewing machine and hence a slack portion formed on a lower portion of cloth for engaging with the forward end of a shuttle body disappears following an operation of a thread take-up spring pulling up the needle thread when the needle passes through the cloth. This may disadvantageously result in formation of no first stitch (the so-called skipping stitch).
When the sewing machine operating switch is moved to ON without exchanging the needle thread, the needle thread may be in a state disengaging from the thread engaging part of the balance due to slacking during the unused state of the sewing machine. In this case, inconvenience similar to the above takes place to disadvantageously result in the so-called skipping stitch.
An object of the present invention is to provide a sewing machine capable of preventing formation of no first stitch (skipping stitch), which is readily caused immediately after the sewing machine starts to operate.
Another object of the present invention is to readily stop a motor when a needle thread is broken in the aforementioned sewing machine.
A sewing machine according to an aspect of the present invention comprises a thread guide plate having a thread receiving opening, a balance having a thread engaging part and reciprocating through the thread guide plate so that the thread engaging part captures and pulls a needle thread when forwardly moving from left to right and a motor for driving the balance, while the motor is so stopped as to locate the thread engaging part of the balance in the vicinity of a forward movement starting point leftward beyond the thread receiving opening of the thread guide plate.
In the sewing machine according to this aspect, the motor is so stopped as to locate the thread engaging part of the balance in the vicinity of the forward movement starting point leftward beyond the thread receiving opening of the thread guide plate as described above, whereby the thread engaging part of the balance can reliably capture and pull a thread needle also immediately after the sewing machine starts to operate. Therefore, formation of no first stitch (skipping stitch), which is readily caused immediately after the sewing machine starts to operate, can be prevented.
The sewing machine according to the aforementioned aspect preferably further comprises an upper shaft set between the motor and the balance so that the balance reciprocates in association with rotation thereof and an upper shaft rotation detection part detecting rotation of the upper shaft, and the motor is preferably stopped on the basis of a result of detection by the upper shaft rotation detection part. According to this structure, the motor can be readily stopped to locate the thread engaging part of the balance in the vicinity of the forward movement starting point leftward beyond the thread receiving opening of the thread guide plate. In this case, the upper shaft rotation detection part preferably includes a first detected element provided on the upper shaft and a first photoelectric switch detecting the first detected element.
The sewing machine according to the aforementioned aspect preferably further comprises a thread breakage detection part detecting breakage of the needle thread, and the motor is preferably stopped in response to a signal from the thread breakage detection part. According to this structure, the motor can be readily stopped when the needle thread is broken. Thus, the needle thread can be prevented from disadvantageously twining around the thread engaging part of the balance. In this case, the thread breakage detection part preferably includes a rotary part rotating following movement of the needle thread and a rotation detection part detecting rotation of the rotary part. More preferably, the rotary part includes an upper rotator and a lower rotator, the sewing machine further comprises a second detected element provided on a peripheral edge portion of either the upper rotator or the lower rotator, and the rotation detection part includes a second photoelectric switch detecting the second detected element. According to this structure, the rotary part stops rotating when the needle thread is broken, whereby the breakage of the needle thread can be readily detected.
The sewing machine having the aforementioned thread breakage detection part preferably further comprises an upper shaft set between the motor and the balance so that the balance reciprocates in association with rotation thereof and an upper shaft rotation detection part for detecting rotation of the upper shaft, for determining breakage of the needle thread and stopping the motor when the rotation detection part of the thread breakage detection part detects no rotation of the rotary part while the upper shaft rotates by a prescribed number of revolutions. According to this structure, the thread breakage detection part can be prevented from erroneously detecting breakage of the needle thread when the needle thread is not broken. If the needle thread is inferiorly in contact with the rotary part of the thread breakage detection part, the rotary part of the thread breakage detection part may not rotate also when the needle thread is not broken. According to the present invention, such erroneous determination can be effectively prevented by determining breakage of the needle thread when the rotation detection part of the thread breakage detection part detects no rotation of the rotary part while the upper shaft rotates by the prescribed number of revolutions.
In this case, the prescribed number of revolutions is previously set, and the sewing machine preferably further comprises a counter counting up the number of revolutions of the upper shaft and responsively outputting a signal for stopping the motor when the number of revolutions of the upper shaft reaches the prescribed number of revolutions. According to this structure, the motor can be readily stopped in response to the upper shaft reaching the prescribed number of revolutions. In this case, the prescribed number of revolutions is preferably eight.
In the sewing machine including the aforementioned counter, the counter is preferably reset when the rotation detection part of the thread breakage detection part detects rotation of the rotary part. Further, the counter is reset when the motor is stopped.
A sewing machine according to another aspect of the present invention comprises a motor for driving a needle and a thread breakage detection part detecting breakage of a needle thread, while the thread breakage detection part includes a rotary part rotating following movement of the needle thread and a rotation detection part detecting rotation of the rotary part and the motor is stopped in response to a signal from the thread breakage detection part.
The sewing machine according to this aspect is provided with the thread breakage detection part detecting breakage of the needle thread as described above, whereby the motor can be readily stopped when the needle thread is broken. Thus, the needle thread can be prevented from disadvantageously twining around a thread engaging part of a balance when the same is broken.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.